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The boiling, erupting Sun

Bad Astronomy
By Phil Plait
Oct 28, 2010 4:00 PMNov 19, 2019 9:46 PM


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I keep thinking there's nothing new under the Sun-- or on it. With SOHO, and SDO, and a thousand other telescopes pointed at it, it would take something pretty freaking cool to surprise me. Well then. Surprise!

Holy solar retinopathy! That's the Sun? Yup. But this is not a space-based image from some bazillion dollar observatory! This phenomenal picture was taken by astrophotographer Alan Friedman with this relatively small (but very, very nice) 'scope. He shot it on October 20th, and it shows our nearest star in the light of hydrogen, specifically what astronomers call Hα (H-alpha). I'll get to that in a sec... In this picture you can see sunspots, giant convection cells, and the gas that follows magnetic loops piercing the Sun's surface. When we see them against the Sun's surface they're called filaments, and when they arc against the background sky on the edge of the Sun's disk they're called prominences. The image he took is amazingly high-resolution! He has two closeups, one of the filament and sunspot near the edge of the disk on the left, and the other of prominences leaping up off the edge and silhouetted against the sky:

Wow, that's breathtaking! They look so delicate, probably because they make the Sun look fuzzy, like a comfy blanket... but have no doubts on the fury and scale of what you're seeing here. See that little bright spot on the plume on the left, just above the Sun's edge? That spot is the same size as the Earth. The image to the right should make that fairly clear; I made the Earth pretty close to the right size for comparison. Our planet is about 13,000 km (8000 miles) in diameter, so that one minor prominence is roughly 50,000 km high. That's 30,000 miles. And it's positively dwarfed by the Sun itself. A million Earths could fit inside the Sun. In case you woke up today feeling important. I want to note that there's a freaky optical illusion I get when I look at the top picture: if I look at one part, say the right hand edge, then quickly move my eye to the top, it appears as if the whole disk of the Sun shrinks for a moment. It's one of those really weird illusions that's very difficult to pin down. Anyone else see it? Now, about that picture and how it works... The Sun's surface puts out light at all wavelengths, but the surface isn't solid. It's a gas, and it tapers off with height. Normally, a thin gas in space emits light at very specific colors as electrons jump from one energy level to another in the individual atoms. But compressed gas in the thicker, denser part of the Sun mashes together all those energies, spreading them out, so it emits white light (that layer of the Sun is called the photosphere). Above that layer, where the gas is thinner (in a layer called the chromosphere), the hydrogen does emit light at specific colors. One of these, Hα, is in the red part of the spectrum, and in fact hot, thin hydrogen emits very strongly in Hα. By plopping a filter in front of a telescope, you can block a lot of the light from the photosphere but let light from the chromosphere through. That's what Alan Friedman did -- he used a filter that let through a very narrow range of colors centered on Hα -- to get this stunning picture. Well that, plus quite a bit of image processing! But everything you're seeing there is real, and is happening on the Sun. [Update: well, mea culpa: the next paragraph is wrong. I got a note from Alan: he inverted the picture of the disk of the Sun to enhance contrast. I didn't realize this, and assumed it was natural. I knew the Sun was naturally limb-darkened in visible light due to the way gas absorbs and emits light, and also that the chromosphere was thin, so it made sense to me that in Hα the Sun would be limb-brightened. Worse, I've done a lot of work with nebulae and other objects that are thin shells of gas, making them limb-brightened too. All this together led me to write the following paragraph, which turns out to be incorrect! My apologies for that, and hope it hasn't confused anyone! Now I'm off to figure out just why what I said was wrong, and the Sun is limb darkened even in Hα.]

Oh-- see how the Sun gets brighter near the edge and darker in the middle? That's not an illusion, it's real. The gas emitting Hα light is like a thick shell surrounding the Sun's surface (like an atmosphere). When we look straight down on the middle of the Sun we're looking through a few thousand kilometers of it, but as you look closer to the edge you're seeing through more and more of the gas. The more gas you see, the more light it emits, so the edge looks brighter. In white light, the opposite is true; the Sun is dark at the edges... but that's complicated enough that I'll just send you here to find out why.

All in all, an amazing and somewhat unsettling picture of a star seen close up. And it's funny: there's nothing in this picture I didn't know about, or have some familiarity with. But somehow, the way Alan presents it, this picture really is amazingly different. I like seeing familiar things in new ways. It jolts me out of complacency. What more can a skeptical scientist ask for?

Tip o' the sunshade to Jason Major, and of course Alan Friedman for giving me permission to post his images!

Related posts: - SDO lunar transit: now with video! - The Sun rises again - One solar piece of flare - Solar storms coming our way this week? - Two solar ISS transits!

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